vmgenid-test: use boot-sector infrastructure

vmgenid-test: use boot-sector infrastructure

There's no requirement for RSDP to be installed last
by the firmware, so in rare cases vmgen id test hits
a race: RSDP is there but VM GEN ID isn't.

To fix, switch to common boot sector infrastructure.

Cc: Laszlo Ersek <[email protected]>
Cc: Peter Maydell <[email protected]>
Cc: Ben Warren <[email protected]>
Signed-off-by: Michael S. Tsirkin <[email protected]>
Reviewed-by: Ben Warren <[email protected]>
Message-id: 1500046217[email protected]
Signed-off-by: Peter Maydell <[email protected]>